There has been a need to increase the stability of polarizing materials against degradation due to heat and chemical exposure, in order thereby to produce superior polarizing materials. These materials, preferably in the form of colloidal particles, can be employed as the suspended material in a fluid suspension for use as the working material in a light valve.
Light valves incorporating fluid suspensions have been known for many years. Fluid suspensions of herapathite in a suitable liquid have heretofore been commonly preferred, although other types of particles have been suggested. In general, the shape of the particles used in such a light valve should be such that in one orientation they intercept more light than in another orientation. Particles which are needle-shaped, rod-shaped, lathshaped, or in the form of thin flakes, have been suggested. The particles may variously be light-absorbing or light-reflecting, polarizing, birefringent, metallic or non-metallic, and the like. In addition to herapathite, many other materials have been suggested such as graphite, mica, grarnet red, aluminum, periodides of alkaloid sulphate salts, etc. Preferably, dichroic, birefringent or polarizing crystals are employed.
Very finely divided or minute particles, preferably collidal, are employed and are suspended in a liquid in which the particles are not soluble, and which is of suitable viscosity and relatively high electrical resistivity. In order to help stabilize the suspension when in the non-actuated state, a protective colloid, preferably a polymer, should be used to prevent agglomeration or settling.
Both electric and magnetic fields have been suggested for aligning the particles, although electric fields are more common. To apply an electric field, conductive area electrodes are provided on a pair of oppositely disposed walls of the cell, and an electric potential applied thereto. The electrodes may be thin transparent conductive coatings on the inner sides of the front and rear walls of the cell, thereby forming an ohmic type cell wherein the electrodes are in contact with the fluid suspension. It has also been suggested to cover the electrodes with a thin layer of transparent material such as glass in order to protect the electrodes. Such thin layers of glass form dielectric layers between the electrodes and the fluid suspension, and the cells may be termed capacitive cells. Direct, alternating and pulsed voltages have been applied to the electrodes in order to align the particles in the fluid suspension. When the voltage is removed, the particles return to a disoriented random condition due to Brownian movement.
Commonly the front and rear walls of the cell are transparent, for example, panels of glass or plastic. With no applied field, and random orientation of the particles, the cell has a low transmission to light and accordingly is in its "closed" condition. When a field is applied, the particles become aligned and the cell is in its "open" or light transmitting condition. Instead of making the rear wall transparent, it may be made reflective or a reflective layer may be placed behind it. In such case light is absorbed when the cell is unenergized and is reflected when the cell is energized. These principal actions may be modified by employing light-reflecting rather than light-absorbing particles.
As aforesaid, one of the most common materials heretofore used in light valve suspensions is herapathite as disclosed in the Land patents, U.S. Pat. No. 1,951,664 and U.S. Pat. No. 1,955,923. Herapathite is quinine bisulfate periodide, the formula for which is stated in the Merck Index (Eighth Edition) as 4C.sub.20 H.sub.24 N.sub.2 O.sub.2.3H.sub.2 SO.sub.4.2HI.I.sub.4.6H.sub.2 O. The Merck Index is published by Merck & Co., Inc., Rahway, N.J. Herapathite, although an effective polarizing material, is not stable either to heat or to small or even trace amounts of certain types of chemicals. Because, in many uses, suspensions are subject to exposure to either or both the aforesaid conditions, it is important and usually essential that the particles and suspension not be subject to degradation due to heat or exposure to chemicals during or after their formation.
It is particularly important to avoid deterioration of particles and suspension, so that the suspension can be used as the working material in light valves. For example, it has been observed that a suspension of herapathite particles suspended in isopentyl acetate liquid or other similar liquid esters, together with the polymer nitrocellulose which is used to help keep the particles suspended in the manner of the prior art, will change color from blue initially to red-purple after a period of several months, even at room temperature. At higher temperatures the color degradation may be even more severe and takes place much more rapidly.
Also, in order to prepare a suspension of herapathite or the improved materials of this invention, the particles must be prepared in the presence of a solvent, some of which solvent may remain in trace amounts in the final fluid suspension. If such chemical solvents degrade the suspended particles, as is evidenced, for example by a color change or a loss in the optical density of the suspension, the particles and suspension are unlikely to be commercially usuable over a long period of time. Accordingly, herapathite has limitations for its use in a light valve suspension because it partially decomposes when in contact with common solvents such as methanol, and 2-ethoxyethanol, which solvents or others similar to them are often necessarily present during particle formation. Degradation products of nitrocellulose such as nitrous acid also seem to attack herapathite.